What are some tips for swimming freestyle faster and more efficiently?

This is a very complex question with millions of answers. You can look at it from 2 perspectives: a) assuming you are already a competitive swimmer with a technique, you should probably hit the gym to gain some muscle mass, increase the strength in your core body and work on streamlining your body better b) assuming you swim just for fun and you just want to learn the right technique which in turn will make you swim faster, then you should focus on first slowing down, going back to the basics and getting your body well aligned (body/head position) and learning the principles behind swimming. When you master this, you can move on to more complex things like improving your kick and your underwater pull. To get you started, perhaps this will help to get you started: http://blog.swimator.com/2011/01/how-to-swim-faster-easier-learning-to.html

For me personally, learning to 'trim' my body was a big boost. By trimming I mean learning to keep your body perfectly flat in the water. Different people have different builds and so lie differently in the water. You can use your head position to trim your whole body. For example, my legs tend to sink, which is bad.  By looking straight down in the water at the bottom of the pool, my legs come up, and I'm then nice and flat. Some people have very buoyant legs and kick a lot of air, (lots of splashing) which is also bad.  They're natural position is 'tilted' the opposite way to me, with the head down below the legs .  By looking up slightly (at the far wall) the legs come down and they become flat.  Experiment with head position until your heels are just breaking the surface as your swim. I'm not a swim coach, this is from my own observation, but it signifcantly speeded up my swimming.

I couldnt have said it better myself, just work on your kick and rotating your hips with your arms. But Libor Janek has it all pretty much there. Good luck!